In conventional technology, coal is used to produce coal gas, natural gas, or used to produce gas by coking at high temperature, medium temperature or low temperature. However, the above-mentioned technology is required to form pulverized coal into lumps or sift lump coal, which increases the cost of raw material, or result in the produced gas without a high heat value, a big additional value, and a significant economic and social benefits.
The heating mode of furnace can be classified as external-heating mode, internal-heating mode and hybrid-heating mode. The heating medium in external-heating furnace is not contact directly with raw materials and heat is transferred from furnace wall. The heating medium in the internal-heating furnace contacts with the raw materials directly, and the heating methods are classified as solid heat carrier mode and gas heat carrier mode according to different heat mediums.
The method in internal-heating mode and gas heat carrier mode is a typical method used in the industry. This method uses a vertical continuous furnace in internal heating mode and gas heat carrier mode, which includes three parts from top to bottom: a drying section, a decomposition section and a cooling section. Lignite coals or their compressed blocks (about 25˜60 mm) move from top to bottom to countercurrent contact with the combustion gas directly so as to be heated for decomposition at low temperature. When a moisture content of raw material in furnace roof is about 15%, the raw material should be dried in the drying section to attain a moisture content below 1.0%, and the upstream hot combustion gas at about 250 degrees centigrade is cooled to a temperature at 80˜100 degrees centigrade. Then, the dried raw material is heated to about 500 degrees centigrade by the oxygen-free combustion gas at 600˜700 degrees centigrade in the decomposition section to be decomposed; The hot gas is cooled to about 250 degrees centigrade, and the produced semi-coke is transferred to the cooling section and cooled by cool gas. Thereafter, the semi-coke is discharged and further cooled by water and air. The volatiles escaped from the decomposition section are subjected to condensation and cooling steps, etc to attain tar and pyrolysis water. This kind of furnace has ever been built in Germany, United States, Soviet Union, Czechoslovakia, New Zealand and Japan.
The method in internal-heating mode and solid heat carrier mode is a typical method of internal heating style. The raw materials are lignite coal, non-caking coal, weakly-caking coal and oil shale. In the 1950s, there is an intermediate testing apparatus built with a processing capacity of 10t/h coal in Dorsten of Federal Republic of Germany, and the used heat carrier are solid particles (small ceramic balls, sands or semi-cokes). Since the process product gas does not include exhaust gas, the equipment for later processing system has a smaller size and the gas has a higher heat value up to 20.5˜40.6 MJ/m3. The method has a large processing capacity because of its large temperature difference, small particles and fast heat transfer. The resulting liquid products constitute a majority and the yield can be up to 30% when processing high-volatile coal. The technical process of L-R method for low-temperature coal decomposition is firstly mixing the preheated small blocks of raw coals with the hot semi-coke from separator in the mixer so as to initiate a thermal decomposition. Then, they are falling into the buffer, and staying a certain time to complete the thermal decomposition. The semi-cokes from buffer come into the bottom of a riser, and are transmitted by hot air and burned off the residual carbon therein in riser at the same time so as to raise the temperature, and then the semi-coke is introduced into the separator for gas-solid separation. After that, the semi-cokes are returned to the mixer, and so circulated. A high heat value gas can be attained from the escaped volatiles from the mixer after dedusting, condensation, cooling and recycling oils.
At present, there are two kinds of conventional coal decomposition apparatus, one of which has an up-draft kiln structure. The up-draft kiln structure is used for combusting flue gas and combustible gases produced by coal, which has low gas purity and a low additional value, as well as partially discharge of gas. This results in a significant resources wasting and environmental pollution. Another kind of coal decomposition equipment has a shaft kiln structure. In the structure, coal lumps are placed on clapboard with holes, and a heater is provided above the coal lumps. Because the coal lumps on the clapboard are accumulated to a certain thickness, so they cannot be uniformly heated and decomposed, and are required to be cyclically heated and decomposed by the decomposed gas, wherein coal lumps are decomposed with a lower rate than that of pulverized coal . More importantly, since the presence of large amount of holes for ventilation and circulatory function provided on the clapboard, pulverized coal can leak through the holes. To avoid this, it is necessary to process the pulverized coal into coal briquette when introducing it into the shaft kiln. Thus, it will increase the cost of pulverized coal decomposition, and reduce the economic benefits because the pulverized coal cannot be directly used for coal decomposition in up-draft kiln.